A self-alignment type semiconductor laser provided with a light confinement effect due to a current constriction layer has such a construction as shown in FIG. 4 as an example. That is, as shown in FIG. 4, on a substrate 21 made of, e.g., an n-type GaAs are sequentially grown epitaxially an n-type clad layer 22 made of, e.g., n-type Al0.6Ga0.4As, an active layer 23 made of non-doped Al0.2Ga0.8As, a p-type first clad layer 24a made of p-type Al0.6Ga0.4As, an etching stopping layer 25, a current constriction layer 26 made of, e.g., n-type Al0.7Ga0.3As, a p-type second clad layer 24b made of p-type Al0.6Ga0.4As, and a p-type contact layer 27 made of GaAs, on the top surface of which is formed a p side electrode 28 and on the back surface of the GaAs substrate 21 is formed an n side electrode 29, so that the resultant wafer is subdivided into chips by cleavage or the like to thereby form a semiconductor laser (LD) chip having a construction shown in FIG. 4.
This construction employs, to use the laser for write-in operations or the like, a method for disposing the current constriction layer near the active layer or enlarge the mixed-crystal ratio of Al in the current constriction layer to provide an effective difference in refractive index in order to enhance the light confinement effect, thus oscillating a laser at a high power.
Also, to use the laser for read-out operations or the like, such a method is required to be employed that the noise is reduced at a low power and, for self excitement, the mixed-crystal ratio of Al in the current constriction layer is reduced or the current constriction layer is disposed distant from the active layer to thereby relax the light confinement effect in order to spread the light, thus enabling forming a supersaturating absorption layer outside a current implanting region in the active layer.
As mentioned above, in order to enhance the light confinement effects and oscillate a laser at a high power, the current constriction layer must be disposed near the active layer as much as possible or the mixed-crystal ratio of Al in the current constriction layer made of an AlGaAs-based compound semiconductor must be enlarged to thereby reduce the refractive index. If the mixed-crystal ratio of Al is enlarged, however, the exposed surface of the current constriction layer after a stripe trench is formed therein easily corrodes because Al is very easily oxidized, thus suffering from a problem that a clean mono-crystal semiconductor layer cannot easily be grown when a semiconductor layer is grown again. Although by, in particular, forming beforehand a protective layer such as made of GaAs on the top-most surface of the current constriction layer, thermal etching can be carried out before the re-growing, thus providing a clean layer, the side walls of the stripe trench cannot be cleaned in such a way, so that the semiconductor layer is liable to be poly-crystallized to thereby flow a leakage current and so increase the threshold current value, thus leading to a problem of an increased electric resistance due to poly-crystallization and also a rise in the operation current.
Since the current constriction layer, on the other hand, has a conductivity type different from that of its surrounding clad layer to thereby prevent a current flow by the reverse-biased pn junction, the pn-junction portion has a depletion layer formed thereon due to the reverse biasing, so that as shown in FIG. 5, if the current constriction layer 26 is formed too close to the active layer 23, the depletion layer (refer to C in FIG. 5) reaches the active layer 23. If the depletion layer C reaches to the active layer 23, as shown in FIG. 5, a current I flows to the portion of the current constriction layer 26 where no stripe trench is formed to disable from constricting the current, thus leading to a problem that the invalid current flows through the active layer.
Although to use a laser for both write-in and read-out operations, on the other hand, the laser must oscillate at a high power without generating a kink while being self-excited at a low power to thereby reduce the noise, as mentioned above, there is a trade-off relationship between self-excitement at a low power and obtaining a high power and, therefore, both requirements cannot be satisfied at the same time, so that a self-excitement type semiconductor laser, which has a large fluctuation in power, suffers from a phenomenon called a kink that the power drops during the process of increasing the operation current, leading to a problem of difficulty in obtaining of a high power. A semiconductor laser for obtaining a high power, on the other hand, cannot be self-excited, leading to a problem that the noise cannot be suppressed.
In view of the above, it is an object of the invention to provide a self-excitement type, high-power semiconductor laser that can operate in a stable manner even at a high power without generating a kink while being self-excited at a low power.
It is another object of the invention to provide a high-power semiconductor laser which can operates in a stable manner even at a high power with no kink generated without enlarging so much the mixed-crystal ratio of Al in the current constriction layer and also with preventing a depletion layer due to a pn junction of the current constriction layer from reaching the active layer.